1. Field of the Invention
The present invention relates generally to an electrochemical cell including an anode half-cell with an anode, a cathode half-cell with a cathode and an ion-exchange membrane arranged between the anode half-cell and the cathode half-cell, wherein the anode and/or the cathode is a gas diffusion electrode. The instant invention furthermore relates to processes for electrolyzing an aqueous solution of alkali chloride.
2. Description of Related Art
WO 01/57290 discloses an electrolysis cell with a gas diffusion electrode, in which a porous layer is provided in a gap between the gas diffusion electrode and the ion-exchange membrane. Under the effect of gravity, an electrolyte flows downwards from above via a porous layer through the gap. The porous layer according to WO-A 01/57290 may include foams, wire meshes or the like.
U.S. Pat. No. 6,117,286 likewise describes an electrolysis cell with a gas diffusion electrode for electrolyzing a sodium chloride solution, in which a layer of a hydrophilic material is located in a gap between the gas diffusion electrode and the ion-exchange membrane. The layer of hydrophilic material preferably has a porous structure, which contains a corrosion-resistant metal or resin. Meshes, woven fabrics or foams may be used as the porous structure. Sodium hydroxide, the electrolyte, flows downwards under gravity via the layer of hydrophilic material to the bottom of the electrolysis cell.
EP-A 1 033 419 furthermore discloses an electrolysis cell with a gas diffusion electrode as the cathode for electrolyzing a sodium chloride solution. In the cathode half-cell, the electrolyte, which is separated from the gas space by a gas diffusion electrode, flows downwards. A hydrophilic, porous material through which the electrolyte flows is also provided. Porous materials disclosed include metals, metal oxides or organic materials, provided that they are corrosion-resistant.
Electrolysis cells with a gas diffusion electrode from the prior art, generally do not ensure that the gap between the gas diffusion electrode and the ion-exchange membrane can be completely filled with electrolyte due to the fact that the porous material is present. This is disadvantageous because as a result, zones arise in the gap and gas forms therein and accumulates. No electric current can flow in these zones. Thus, electricity flows exclusively through electrolyte-filled zones in the gap, resulting in a higher local current density, which in turn gives rise to a higher electrolysis voltage. If the gas collects on the ion-exchange membrane, the membrane will not be completely saturated and may be damaged due to the absence of electrolyte.
The use of porous layers furthermore has the disadvantage that any gas which has entered the porous structure can only get back out again with difficulty. Thus gas can accumulate within the porous layer, and as such gives rise to the above-stated disadvantages. Under operating conditions, gas from the gas space can also pass out from the gas space through the gas diffusion electrode and into the gap. Gas diffusion electrodes furthermore have a tendency to allow increasing quantities of gas to pass through at unsaturated points, and as a result the effect is amplified.